1. Field of the Invention
The present invention relates to an NO.sub.x -cleaning paving block, and more particularly to an NO.sub.x -cleaning paving block which has an enhanced NO.sub.x -cleaning capability because of an increased efficiency of fixing NO.sub.x from the air and an increased pluvial NO.sub.x -cleaning efficiency, thus helping to protect the environment, and is provided with a non-slip property, wear resistance and further a very decorative property.
2. Description of the Background
Air pollution because of NO.sub.x which is present in exhaust gas discharged from automobiles, particularly diesel motorcars, remains a problem because of the ever increasing number of automobiles in use, because of increased traffic congestion, and the like.
Known NO.sub.x concentration-lowering materials include metal oxides, of which titanium dioxide is known to have a strong photocatalytic action. Research into removing NO.sub.x by the strong photocatalytic action of titanium dioxide has attracted increasing interest in recent years, and the removal of NO.sub.x from the atmosphere upon contact of the NO.sub.x with the external walls of buildings constructed of sheets or panels which incorporate titanium oxide therein is nearing the commercial stage.
The present inventors have conducted research on sheets and panels which can be used for constructing the external walls of buildings, particularly in view of the conditions under which they can be used for paving purposes, and have found that a paving block formed of a combination of a dough comprising cement, titanium dioxide powder and sand, and a concrete base functions as a NO.sub.x -cleaning paving block (interlocking block) which efficiently removes NO.sub.x over time without deterioration of the catalytic property of titanium dioxide, and has an excellent non-slip property and wear resistance suitable for paving purposes. It also has a very decorative property. The NO.sub.x -cleaning paving block has been studied from various aspects in order to increase its performance, with the findings that:
1) the surface layer material must be exposed to a maximum dose of ultraviolet light; and PA1 2) the NO.sub.x gas which evolves during the night must be temporarily adsorbed on an adsorbing material from which the gas must be removed during the day. These findings are at the base of the present invention.